Centralized switching arrangements with inward dialing



July 25, 1967 J. s. YOUNG 3,333,052

CENTRALIZED SWITCHING ARRANGEMENTS WITH INWARD DIALING Filed April 14, 1964 1l Sheets-Sheet l J. S. YOUNG July 25, 1967 CENTRALIZED SWTCHING ARRANGEMENTS WITH INWARD DIALING ll Sheets-Sheet 2 Filed April 14. 1964 TRK.

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July 25, 1967 J. s. YOUNG 3,333,062

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FIG IO July 25, 1967 J. s. YOUNG 3,333,062

CENTRALIZED SWITCHING ARRANGEMENTS WITH INWARD DIALING Filed April 14, 1964 1l Sheets-Sheet ll CDNCENTRATOR LINK p -GROUP A GRPA START DIST) FIG II 730 TRUNK POSITION 2DOIII7 FINDER FINDER 75o I GPI DIDSETQKNGCKT. E] [E POS. RELAY I z z EQUIP FOR CUSTOMER 2 E E T CUSTOMER 2 2OOI2I z DID TRK CKT f -CONCENTRATOR POS RELAY LINK CRP D SERVING 730 START GDIQSPT EQUIP FOR 1 M CUSTOMER I TRUNK POSITIONJ L CUSTO ERI FINDER FINDER GP E] [E 750 POS.SEL.Sw. POS BUSY CALL f I 'A V AWAITING I /TEST LEADS SIGNAL7 VI- I -l FLE CUST. Ip.- CUST START POS. IDLE I 62o CKT. TEST RELAYV A I TRK,FINDER l 705| PMI POS. I' l I BANK MARKINGI Pl Lf-'ED/FL I ;D l/CUST START 62o 1 "4 4 CKT. CUST. 2

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United States Patent 3,333,062 CENTRALIZED SWITCHING ARRANGEMENTS WITH INWARD DIALING John S. Young, Addison, Ill., assiguor to Automatic Electric Laboratories, Inc., Northlake, lll., a corporation of Delaware Filed Apr. 14, 1964, Ser. No. 359,626 16 Claims. (Cl. 179-27) ABSTRACT F THE DISCLOSURE A centralized telephone switching system using automatic switching apparatus for completing direct inward dialed calls from local or distant subscriber stations to a plurality of PABX groups, each having a plurality of subscribers and a plurality of attendant positions, and to regular central oce non-group subscribers. An incoming connection to a called PABX subscriber is established over a direct inward dialing trunk circuit and switch train, with a marking corresponding to the group identity of the called subscriber being returned back over the switch train to the trunk circuit. A call transfer circuit is taken into use in response to receipt of a call forwarding sigial initiated 'by the called subscriber, with a group discriminating circuit controlled by the group identity markin-g to allow the transfer circuit to establish a transfer connection only to another subscriber, or attendant, of the same group identity. The call may be consecutively transferred to any number of other subscribers, or [an attendant position, having the same group identity by using additional transfer circuits. Time division multiplex signalling apparatus is provided for controlling the establishing of all transfer connections over a single conductor in the switch train by an attendant, an originally called subscriber and a subscriber to whom the original call was transferred or retransferred. The time division signalling apparatus is also used in one instance for signalling the attendant of an incoming call and the type, or class, of service; and in another instance for transmitting campon-busy or busy-cut-in signals under control of the attendant.

This invention relates to telephone switching systems in ygeneral and more particularly to improved automatic switching arrangements for establishing direct inward dialed connections to stations in a telephone exchange area. The term stations, as used in this specification, broadly encompasses telephone users receiving direct in ward dialed calls originated locally or from distant points in a nationwide telephone system. These stations may include substations and attendants of private automatic branch exchange (PABX) groups and, optionally, a group or groups of regular stations in a telephone eX- change area.

In the past it has been customary to supply separate PABX switching equipment for setting up direct inward and dialed and transfer calls to PABX substations of each customer group. Generally speaking, the switching equipment was located on the PABX customers premises and interconnected with the central exchange switch trains by trunk lines to which outsiders were connected when making calls to the PABX substations. When inward dialed calls would not be completed automatically, in the preferred manner, facilities were provided for routing the incompleted calls to an attendant, who would then manually complete the call to the desired substation.

As an alternative measure to the above mentioned technique of using individual PABX switching arrangement, it has been proposed to centralize the switching equipment in the central telephone office. This results in imlCC arrangement, it is advantageous to terminate PABX sub-` stations at the central ofce in the manner of regular community area stations, that is on line relay equipment terminating line conductors from standard subsets. As a part of this switching plan, each station group of a dilferent class of service, namely the regular stations and the PABX stations, had their own register yand marker equipment. The register and marker equipment of each PABX group included provisions for counting and storing dialed digits, thereby distinguishing between calls to be permitted or denied. A switching plan of this nature is relatively complex and, generally speaking, is confined to situations where the concentrated switching equipment is located in the central office.

The above mentioned changes taking place in a community area may not have been anticipated in the planning of the trunking and the layout of equipment in the central telephone oice serving that area. For instance the area may be newly developed and include buildings housing private branch exchange customer groups as well as regular telephone customers, either business or residential.

It is therefore an object of this invention to provide an improved automatic telephone switching system with an arrangement permitting direct inward dialed and related telephone calls to telephone stations regardless of whether the concentrated switching equipment is located in the central office or at a distant point from the central office. Such a distant point may be at the PABX customers premises.

In view of what has been said above, it is a further object of this invention to provide a unitary system for establishing direct inward dialed calls to groups of telephone stations including a plurality of groups of private automatic branch substations and, optionally, a group of regular stations. And furthermore, the switching equipment may be located either in the central exchange oice or at the community area.

It is also an object of this invention to provide a direct inward dialing switching system suited to the assignment of substations numbers on a decimal basis. In such a system, station assignments are made in blocks of according to the ranks of the switch train. That is in `groups of 100, 1000, etc.

Accordingly, another object of the invention is to prof vide a switching system wherein the last rank of the switch train used in completing direct inward dialed calls to private automatic branch exchange substations can be shared for local group telephone service.

It is also an object of this invention to permit private branch exchange substations, reached by direct inward dialing call methods, to control the establishment of callforwarding connections to other substations of that PABX group, thereby minimizing the number of attendant completed calls.

A further object of this invention is to provide in a switching system an arrangement whereby stations of any group, which may also include the regular exchange group, are prevented from establishing transfer connections to stations of other groups served by the switching system.

A principal feature of the invention relates to a ccntralized switching system with improved circuit arrangements for the establishment of inward dialed connections to groups of stations, facilities being provided whereby the stations of some of these groups may set up transfer connections to other stations in their own group.

One of the features of the invention relates to the provision of group identification facilities, marked in the setting up of the inward dialed connections, for confining the establishment of PABX substation-originated transfer connections to substations of the identified group. This may apply to successive transfers of a given call, when required.

Another feature of the invention relates to an arrangement wherein, on inward dialed calls, the trunk circuit is originally transparent and thev connector split Subsequently this condition is reversed, that is, the trunk circuit is :split and the connector rendered transparent. In this fashion, pulse repetition in the setting up of inward dialed and transfer connections is avoided, and transmission between the parties of a call improved.

Yet, because of the above-mentioned retermination of the called loop in the trunk circuit, back dialing of transfer signals by a called substation is facilitated.

In the embodiment described, on an inward dialed call to a station, a two-step relay in the trunk circuit is operated to its rst step, upon answering by the called substation, to forward a signal causing the connector Ito switch the called station loop physically through to the trunk circuit. The relay is then operated to its second position to complete the connection of the called station-be it a PABX substation or a regular central oice stationto a split battery feed inthe trunk circuit.

A further feature of the invention relates to circuit arrangements wherein the last-mentioned regular central office sta-tions, although switched through to the trunk circuit split battery feed, are kept from effecting call transfers of any sort. This is automatically brought about by the V,aforementioned marked group identification facilities which prevent seizure of the call-forwarding equipment on such calls.

Another feature of the invention relates to a circuit arrangement wherein answer supervision is returned by the trunk circuit on inward dialed calls to stations but is prevented on PABX group number calls (listed number calls) until the call has been answered by an attendant. A ringing signal is returned to the calling party until the call has been answered by the attendant.

A further feature of the invention is in the trunk circuit arrangement whereby directory listed number calls to a PABX group are automatically routed over a call-forwarding switch train to an 4attendant of that group. Discrimination between listed number callsand calls to stations is effected according to the duration of the an-swer supervisory signal returned by the connector to the trunk circuit. After this the switch train between the trunk circuit and the listed number trunk, as well as the last-mentioned trunk itself are released. A listed number trunk returning a momentary answering supervisory signal has been disclosed in U.S. Paten-t 3,033,938 issued to John S. Young on May 8, 1962. Y

Timing means are also provided in the direct inward trunk circuit to prevent substation-originated transfer signals from affecting supervision to the outside caller, and to prevent response to a transfer signal if the called party fumblesV the handset when answering the call.

A still further feature of the invention relates to a exible switching arrangement wherein use is made of the novel time division signaling facilities disclosed in the copending iU.S. PatentV application Ser. No. 295,098 of Robert M. Schildgen and John S. Young filed July 15,

1,963, for controlling the establishment of calls forwarded Vto PABX substations by the attendant or the originally called substations. This technique is also used in controlling the establishment of all call-forwarding connections vthereafter origina-ted by a substation to whom the original call was transferred, or re-transferred. In this manner a considerable number of these controls can be executed over the same conductor, for example an extra-control conductor, of the switching equipment in question. Time division signals, assigned in diiferent recurring time slots to identify the particular station group preferable are also employed for enabling, or denying, the establishment of station-originated connections according to the matching, or mismatch, or corresponding signal output terminals of the time division power supply. Y

A feature of the invention relates to provisions in the inward dialing trunk circuit or a two step relay controlled both in the establishment of inward dialed calls and in the case of automatic forwarding of listed number calls to 'the attendant. On inward dialed Icalls the relay is operated to the first step, and is permitted to operate -to its second step in response only to the receipt of a dialed-back digit greater than 1 to forward the-call to -the attendant. If, on the other hand, the digit 1 is Vdialed the relay cannot operate to its second step and this permits seizure of the transfer link and the remainder of the switch train used in forwarding the call to a substation. It may be mentioned at this point that such forwarding may beY elected for purposes of a consultation, of a limited or 3-.party conference (the originating substation dials a digit after the other substation has answered) or a transfer the originating substation disconnects). Y

According to another feature of the invention, the physical connections from called substations to the inward dialing trunk circuit are automatically extended to the switches successively seized in the forwardly of a ca ll to another substation; after the call lhas been answered, the connections are re-terminated in the battery feed relay on the calling side of the transfer link used in the forwarding connection.

It is this battery feedY relay which responds to a digitdialed by the rst substation for au-tomatically Vcausing the outside party to be connected into a limited conference with both substations. On the other hand, disconnection of the above-mentioned first substation causes the transfer link .to automatically forward a signal to the connector switch used in the forwarding (transfer) connection thereby physicaly switching through the second-called substation loop to the aforementioned battery feed relay in the inward dialing trunk circuit. This conditions the ytrunk, -circuit for the receipt of transferinitiating signals by the second-called substation.

According :to another feature of the invention the call forwarding ararngement described lherein, the second. called PABX substation is enabled to in turn establish a forwarding cal to a further substation, namely by seizing a second transfer link in addition to the one over which the second-called substation received the'call.

A further feature of the invention relates to a call distribution arrangement wherein trunk circuit seizure of equipment for forwarding initiated substation and listed number calls to an attendant of an identified group depends upon the availability of an idle circuit for terminat-4 ing the call.

More particularly, ,the call-forwarding equipment includes group start circuits, one for each substation group which are connected to all trunk circuits, `and the above-l mentioned seizure of the call-forwarding equipment is made dependent upon return -to the trunk circuit from the start circuit of the identified group of a marking -ground indicating an open gate for `receiving the call being forwarded.

In the embodiment of the invention described herein, the call-forwarding equipment further includes a concentrator link allotter seized by the start circuit, each said concentrator link including a trunk finder and position finder, A rotary switch forming part of this allotter is caused to automatically search for an idle attendants position of the group in question. The switch provides a marking of the attendants position on the banks of the position finder switches.

The above-mentioned rotary switch is also instrumental in marking the trunk circuit used in this call on the banks of the trunk finder switches according to a marking potential extended through the group identification circuit.

The concentrator link allotter also has a second rotary switch for selecting an idle concentrator link. This lastmentioned switch has rotary switch bank levels through which controls are effective for causing the position finder and trunk nder of the selected link to iind the above-mentioned finder bank markings.

According to a further feature of the invention, the above-mentioned time division signaling technique is used to signal a call and also the type of call, for example a listed number call or a transfer call, before the attendant. The attendant, in completing the last-mentioned call, operates a dial-in key and thereby automatically seizes a transfer link in preparation for the transmission to said link of dial impulses for the purpose of forwarding the connection to the desired substation. These are the same transfer links that are used by substations in transferring calls to other substations. The above time division signaling arrangement brings about seizure of the above-mentioned transfer link under the control of the attendant.

The concentrator link is released by attendant operation of a trunk release key. Following this release the connection is completed between the outside party and the substations last called over the transfer link.

If the attendant is unable to complete a call because the called line is busy, she can momentarily operate a signal key and cause the connector switch reached over the transfer link to camp-on the busy line. If the key is operated the second time, the connector is caused to cut-in on the busy line.

The time division signaling technique referred to above is also instrumental in the transmission under the attendants control, of the foregoing camp-on or cut-in signals.

Further features of the invention relate to the particu lar arrangements of the elements thereof, whereby, the above-outlined and additional operating features are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following detailed description of a preferred embodiment, taken in conjunction with the accompanying drawings. 'Ihese drawings, FIGURES 1 to 13 inclusive, show, by means of the usual circuit diagrams, a sufficient amount of apparatus to enable the invention to be described and understood.

FIG. 1 shows the schematic diagram of an illustrative example of a system according to the invention. The left hand portion of FIG. l shows a direct inward dialing trunk circuit 200 terminating incoming connections from automatic switching equipment, which may be incoming iifth selectors and banks, or two-conductor repeaters, in a central office of a nationwide telephone systems not shown. The operation of the inward dialing trunk circuit is the same in either case, therefore, the switching equipment shown on the FIGURE 1 can be located in the central otiice or at an outlying location. The inward trunk circuit is connected to a succeeding rank of automatic switching equipment, such as an incoming fifth selector 500, responsive to direct dialed impulses to set up connections to switch-through connectors 5116, 515 for establishing calls to central office stations, substations of PABX groups, or to listed number trunks 516 all as shown at the right hand portion of FIG. l. The regular subsets used at these stations and substations are connected to subscriber lines terminated on line circuits at the switching equipment. The incoming fifth selector switches, shown at the upper portion of FIG. l, have group identification marking conductors extending to the group identification circuit 610 at the left of FIG. l, shown as a portion of the direct inward dialing trunk circuit. The upper block portion of FIG. 1 shows first call-forwarding equipment, here and after referred to as transfer links 100, each including a transfer relay group 493 connected, on the left, to a transfer-finder 491, and on the right to a transfer-selector 509.

The transfer-selector switches have banks connected in multiple with the banks of the incoming fifth selectors and therefore have access to the switch-through connectors for establishing calls to stations except as hereafter described. Above the transfer line there is shown a transfer link allotter 492, with an allotter switch for allotting an idle transfer iinder to find the inward dialing trunk circuit requesting service. The remainder of the main body of FIG. l shows the second call-forwarding equipment hereafter referred to as concentrator links 110 for forwarding listed number and substation-controlled calls to the attendant 1000. The attendant may extend these calls to a substation by dialing back over the concentrator link 110 and the inward dialing trunk circuit 200 into a transfer link which in turn gives access to the switch-through connectors 51S.

In the case of listed number calls or of calls to be forwarded by a PABX substation to an attendant which calls will be described in detail herein below, a gating arrangement in the respective customer start circuit 620 permits the inward dialing trunk circuit to seize a concentrator link, if an attendants position circuit 900 of the PABX customer group signals the availability of an idle terminating circuit 809. In that event, the concentrator link allotter 720 assigns an idle concentrator link 110, each comprising back to back a trunk finder 730 and a position nder 750, to provide a clear path from the inward trunk circuit to the terminating circuit, and hence to the attendant. As shown in detail in FIG. 7, vertical bank markings to the position finder and the trunk finder switches are applied over the banks of rotary switch 7MM of allotter 720. Each bank level of the position finder gives access to the terminating circuits of a separate attendant position. As shown more particularly in FIG. 11 the allotted trunk finder switches, as a matter of lfirst choice, search for marked trunk cir-cuits in the lower levels of the finder bank but will be automaticallyV caused to search inthe upper half of the bank if necessary. Having switched through the connections between the inward dialing trunk circuit and the attendant, the trunk finder and position finder become locally held for the duration of the call.

As mentioned above, the attendant may further extend these calls, that is, listed number calls or substation-controlled calls, by way of the terminating circuit, and the concentrator link used, to the inward dialing trunk circuit, and thence through a transfer link and a switchthrough connector to a desired substation. The group identification circuit portion 610, of the inward dialing trunk circuit 200 insures that the connection will be established only to substations in the group served by the particular attendant.

FIGS. 2 to 10 inclusive show the circuits of the equipment units illustrated in the schematic diagram, FIG. l, in detail. More particularly, FIGS. 2 and 3 combined show the inward dialing trunk circuit details and connections With other equipment units.

FIG. 4 shows the switching arrangements of transfer links whereby calls originated by PABX substations and attendants are completed to other substations of a group.

FIG. 5 shows the arrangements of switch-trains for obtaining access to groups of stations or to a listed number trunk in the case of calls directed to a PABX group attendant.

FIG. 6 shows in detail the circuit wherein the group identity of an inward dialed call is stored for the identification of all calls subsequently originated by stations or attendants for some reason. The figure also shows portions of a start circuit, individual to each PABX customer group, arranged upon seizure by the trunk circuit on a listed number call to initiate the forwarding of the call to an attendant of the identified customer.

FIG. 7 illustrates concentrator type call forwarding equipment comprising an allotter controlling the operations of position finders and trunk finders to provide a direct link between an identified trunk circuit and the corresponding PABX attendant being free to handle the' listed number or substation controlled transfer call of the trunk circuit. Y

FIG. 8 shows the circuit elements required for terminating the above-mentioned direct link paths from the identified trunk circuit establishing calls t-o the attendant.

FIG. 9 illustrates portions of the attendants position circuit connected to other units of the call forwarding equipment.

FIG. 10 shows turret equipment whereby an attendant receives and/or completes inward dialed calls which require personal attention.

FIG. 11 schematically shows the call distribution arrangement involving the concentrator link allotter 720 and concentrator link groups 110, A and B, each serving a different group of trunks and position equipment with preference.

FIG. 12 schematically shows the time division multiplex arrangements used herein for remote control of the operation of various electromechanical relays over a single conductor according to the principles of the abovementioned copending patent application. Closure of a send contact at either end of the EC conduct-or between predetermined circuits of the present system enables a discrete time `ground (TG) signal to be repetitively transmitted over an extra (EC) conductor through the winding of a receive relay to the respective time battery (TB) potential. The corresponding receiving relay is, thereby, caused to operate and it may, if desired, become locally held by another winding.

FIG. 13 is a diagram showing the relative arrangement of FIGS. 2 to 10 inclusive.

Detailed operational descriptions will now follow.

Direct inward dialed calls to substations of a PABX customer group This section of the operational description relates to switching arrangements involved in the establishment of direct inward dialed calls, from local or distant stations, to stations with assigned numbers in a block of 1000 numbers. Particularly, this section relates to switching arrangements whereby inward dialed Iconnections are established to substations of several PABX customer groups. Of the seven digit directory number assigned to the PABX substation, the last four digits, for example, 2322, are significant to the description according to the embodiment of the invention. Impulses of the first digit, 2, set up an access, over the incoming conductors, -lto the DID trunk circuit 200 FIG. 2, and tothe incoming fifth selector switch 500, FIG. 5, digit 3 impulses step the selector wipers 501 to level 3 to seize the idle connector switch 515, and impulses of the remaining digits 22 cause the connector switch to step its wipers to bank terminal connections of the desired substation of the PABX customer group.

The above-mentioned access to the trunk circuit extends a dialing loop, not shown, via the incoming connection to the incoming fth selector switch impulsing relay A, not shown, over a physical path traced from andv -lconductors of FIG. 2 to back contacts 3 and 4 of relay 2RVS, back contacts 1 and 3 of relay ZAT, back contacts 1 and 2 of relay 2SP, over physical conductors -SW and +SW in FIG. 3 to the incoming fifth selector switch 500. The incoming selector is seized and returns ground over the CSW conductor to the trunk circuit FIG.

.3 by Way ofback contacts 4 -of relay 3P to the upper YWinding of relay 3CM. Relay 3CM operates. Ground is also extended back tothe incoming C lead for guarding the trunk circuit 200 against other seizures. The group (300) of the seized connector is identified when the selector 500 steps its Wipers to level 3 at which time the right normal post springs 503 operate and close a path for ground marking through make contacts 504 to the B1 conductor extending from FIG. to the group identification circuit 610 of FIG. 6. This marking ground causes 8 relay 6CU2 to operate and identifythe customer group 1, corresponding to the 300 connector group of the called substation. Relay 6CU2 operated closes la holding path to the trunk circuit 200 through its make contacts 6 over the LK conductor traced back through the trunk circuit to the CSW lead on which ground is standing as has been described above. This holding ground path is traced from the CSW conductor back contacts 6 of relays 2RVS, FIG. 2, through the back contacts 1 of relay 2A and over the LK conductor t-o the group identification relay 6CU2. Obviously the group identitication of the PABX customer no longer depends upon therright normal post springs 503, as operated relay 6CU2 opens the B1 conductor marking path.

As mentioned above, the wipersV of connector switch 515 now engage bank contacts 22" and ringing Vcurrent is transmitted to the called PABX substation. In a well known manner, the connector switch returns reverse battery signals over the -SW and +SW conductors to the trunk circuit 200 when the called substation answers. This reverse battery signal is detected by relay 2SP in the trunk circuit. Relay 2SP operates in the path Vtraced through back contacts 1 and 2 of relay 3SW, make contacts 1, 2 and 3 of Arelay SCM, 'back contacts 1 and 2 of relay 2RVS. This path also includes ,blocking diodes 2D1 and 2D2 in `series with the upper and lower windings of relay 2SP respectively. To the left of these diodes shown on ldrawing FIG. 2 is a voltage divider arrangement consisting of resistors 2R1, 2R2, and 2R3 connected between battery and ground. This voltage divider and diode arrangement insures that relay 2SP will not respond to impulsing on the -SW4 and +SW conductors of FIG. 3 but will be operated by reverse battery from the connector when the called substation answers. Relay 2SP operated, transfers, at its make 'contacts 1 and 2, the incoming conductors from a physical connection previously including lthe incoming selector 500 and connector switch 515 to the upper and lower windings of relay 2A of the trunk circuit 200. Relay 2A operates. When relay 2A operates its make before break contact 1, the guarding ground to the incorning C conductor and the holding ground over the LK lead to the group identification relay 6CU2 no longer depends upon the CSW conductor from the incoming irst selector switch 500, as ground is applied at make contacts 1 of relay 2A. Back contacts 3 of relay 2A open a resistor shunting path across capacitor 2CA5, while make contacts 2 lcomplete a path traced through the make contacts 5 of relay 2SP to a divided circuit, one directly to the upper winding of relay 2RV, the other being to the diode 2D6, capacitor 2CA5, and the lower winding ofkrelay ZRV. Capacitor ZCAS becomes charged, The charging current through the lower winding of relay 2RV opposes the operating current through the upper winding of relay 2RV. Because of this opposed current `condition relay -2RV is slow to operate. With relay 2A operated and relay 2RV not yet operated, a closed path extends from ground through make contact 1 of relay`2A, back contacts 10 of relay 3K, back contacts 2 of relay 2RV, make contacts 5 of relay 2A, resistor 2R5 and the lower winding of relay 2AT to battery. Because of this resistance path relay ZAToperates only to its X contacts, thereby closing its holding path through make X8 contacts. Relay ZAT is thereby preenergized for faster operation to its second step should the substation initiate a call to an attendant 1000. In this type ofdirect inward dialed call the connector switch 515 returns reverse battery for a prolonged period of time enabling trunk circuit relay 2SP to remain operated. The effects of the current through the upper winding of relay 2RV prevail over the charging current through the lower winding, causing relay 2RV .to operate. Slow operating relay 2RVS then operates through the make contacts 1 of relay 2RV, Make 'before break contacts 3 and 4 of operated relay 2RVS complete direct connections from the battery feed relay 2A to the incoming and conductors before opening the previously described operating path for the relay 2A. Relay ZRVS operated also closes its make contacts 6 to complete a path from the ground on the CSW conductor of FIG. 3 through make Contact S of relay 2SP FIG. 2 to the resistor SR6 and the lower winding of relay SSW in FIG. S. Through this resistor path relay SSW operates its X contacts only. Relay SSW operated to the first of its two step positions completes a path for passage of a switching signal from the direct inward dialing trunk circuit 200 to the connector switch 515. In the present embodiment of the invention the switching signal is booster battery transmitted over the path traced from BB and the make contacts 4 of relay 2SP through the X contacts 5 of relay SSW to the CSW conductor of FIG. 3 through the corresponding conductor of the incoming fth selector 500 to the connector switch 515. In response to this switching signal the connector switch disconnects the called PABX substations loop from the battery feed in the connector switch. Before tracing a path from the called loop to the replacement battery feed relay 2E, in the trunk circuit, certain relay operations in the trunk circuit should now be described. It is recalled that relay ZRVS has operated and its back cont-acts 1 and 2 have removed the battery and ground potentials from the voltage divider and diode arrangements connected to relay ZSP, and also that due to the switching through of the connector 51S neither normal battery or reverse battery potentials are on the -SW and -i-SW conductors FIG. 3 of the path traced to the upper and lower windings of trunk circuit relay ZSP FIG. 2. Because these windings are no longer energized, relay ZSP restores, Relay 2SP restored closes a path for the operation of relay SSW to its second step position, that is fully operated. This path is traced from the ground on the CSW conductor of FIG. S through make contacts S of relay SSW to the back contacts of relay ZSP and the lower winding of relay SSW to battery. Relay SSW, being now fully operated, closes the path for the called substation loop traced as a loop on the and -lconductors of the incoming selector switch 500 of FIG. 5 extending back over the -SW, -1-SW conductors to the trunk circuit FIG. 3 where the path continues through make contacts 1 and 2 of the relay SSW, and the back contacts 1 and 2 of relay ZTR to the upper and lower windings of relay 2E FIG. 2. Relay 2E operates and is the called-party side battery feed split to the calling-party side battery feed relay 2A by way of capacitors 2CA1 and 2CA2. Operated relay 2E closes its make contacts 1 and establishes a path for the operation of relay SB. Operated relay SB closes a path for the holding of relay ZRV. This path is traced from ground through make contacts 2 of relay 2A, make contacts 2 of relay SSB, and the upper winding of relay ZRV to battery.

In summation, called REFERENCE A, the following relays are in an operated condition: FIG. 2, relays ZRV, ZRVS, 2A, 2E, and ZAT to its X contacts; FIG. 3, relays SCM, SSW, and relay 3B; and in FIG. 6, relay 6CU2.

Direct inward dialed PABX substation establishes Connections to a second substation As mentioned above, the called PABX substation may call other substations in group 1 for consultation purposes. Another section of this specification will be devoted to the case in which the called PABX substation establishes a connection to the attendant 1000(1). In the case of the call to be described, the first substation party dials the prefix l plus the local directory number of the other party for example the abbreviated number may then be 1323.

With relays operated as listed in reference A, it is known that dialing of the prefix l causes relay 2E of the trunk circuit 200 to restore and later to reoperate. Relay 2E restored closes a path for the operation of the slow releasing relay 2C. This path is from ground at make contacts 2 of relay 2A, back contact 1 relay 2E, back contacts 9 relay 3K, make contact 5 relay 3B, FIG. 3, and through the upper winding of relay 2C to battery. With relay 2E reoperated and relay 2C not yet released a path is closed whereby relay 2TR oper-ates to its X contacts. This path is from ground through make contacts 2 of relay 2A, make contact 1 of relay 2E, back contacts 6 and 11 of relay SK, make contact 4 of relay 2C and then over the TRD Conductor of FIG. 3 extending to the similar conductor in FIG. 6 where the path passes through back contacts 7 of relay 6CUS to conductor TRG extending to FIG. 3, through the back conta-ct 6 of relay SP and the upper winding of relay 2TR to battery. Relay 2C operated has opened the transmission path, at its back contacts 1 and 2, and thereby prevented the prefix digit impulses from affecting the incoming connections to the trunk circuit. A guarding path is provided for holding relay ZTR operated to its X contacts before relay 2C restores its back contacts 3 to set a marking on the vertical bank not shown of the transfer finder switch 491. This path extends from ground at make contacts 2 of relay 2A through make ycontact 2 of relay SB to the left through back contacts 4 of relay ZAT, back Contact S of relay ZTR, and X4 contacts of the same relay to make contacts S of relay SB Vand the upper winding of relay SP in series with the upper Winding of relay ZTR to battery. This series holding path enables relay SP to operate. Operated relay SP opens its back contacts 4 and allows relay SCM to release. Relay 3P operated makes a contact transfer at its make contacts number 6 as will be referred to in a later section of the specification -covering the consultation or transfer of calls to the attendant 1000(1). In that case station-controlled impulsing of relay 2E by digital impulses greater than 1, for instance the digit 0, also causes relay SP to operate and through its make contact 6 to complete a path for the full operation of relay ZAT.

As mentioned above, relay 2C is released, thereby setting up a marking path to the transfer link finder 491 vertical bank, FIG. 4, not shown and in a manner not shown, to the transfer link allotter 492 shown above. This marking path is from ground at make cont-act 2 of relay 2A through make contacts 2 of relay SB, back contact 4 of relay ZAT to relay ZTR back contact S and X4 make contacts, through back contact S of relay 2C to the SLM -marking conductor extending to the vertical bank and alloter of the FIG. 4. Briefly described, the allotter causes an idle transfer finder switch to find the inward dialing tnmk circuit 200 about to make a consultation call to another PABX substation. After finding this trunk circuit, relay 402 of the transfer finder becomes operated and switches through the transmission impulsing and control conductors to succeeding circuits of the transfer link 100. The originally called PABX substation having its loop connected physically to the -SW and -l-SW conductors of FIG. 3 to the battery feed and impulsing relay 2E of FIG. 2 was also connected to the conductors -TN and -l-TN of FIG. S to the bank contacts 404 of the transfer finder switch 491, FIG. 4. Operation of the relay 402 in the finder switch extends the TN and -l-TN conductor loop to the and -lconductors and through contacts of the transfer relay group 49S to the transfer selector switch S09, where the loop terminates in a battery feed impulsing relay not shown in FIG. 5. This loop impulsing path is partially traced as extending through back contacts 1 and 2 of relay 4TA, back contacts S and 4 of relay 4TS, and the back contacts 1 and 2 of relay 4TD to the -L and +L conductors extending to the impulsing relay of the selector switch. Loop seizure of the impulsing relay causes the selector switch to return ground over the C conductor in the normal manner and cause relay 4CC of the transfer relay group 49S to operate. This operating path is through the back cont-acts 3 of relay 4BS, back contact 1 of relay 4CF, back contacts 4 of relay 4S, then the winding of relay 4CC to battery. Operated relay 4CC returns a ground through its make contact 1 to hold relay 402 of the transfer finder switch 491 operated.

contacts of relay 2TR and its lower winding to battery.

YVClosure of this path causes relay ZTR of the trunk Vcircuit to become fully operated to its second step. Operated relay 2TR Lcloses a holding path through its make contacts 8 and the lower winding of relay 3B which had been previously operated. In addition to providing a ground through its make contacts 6 for the'holding of relay 2RV, operated relay 2TR opens its back contacts 1 and 2 and removes battery feed relay 2E from the called PABX substations loop. It is obvious that only the impulsing relay of the transfer selector switch 509, is now directly connected to the called PABX substation.

` Relay 2E restored also permits relay 3B of the trunk circuit 200 to restore.

At this point in the operation of setting up the so called consultation call the selector switch returns dial tone and is ready to accept the remaining digital impulses Yof the called number, that is 323, As a matter of reference, known as REFERENCE B, the following relays are in an operated condition: FIG. 2, relay 2RV, 2RVS, 2A, relay ZAT to its X contacts, relay 2TR; FIG. 3, relay 3SW, and 3P; FIG. 4, relay 4CC; and in FIG. 6, relay 6CU2.

It is recalled that the originally dialed substation, now making the consultation call, was in the PABX customer group number 1 and permitted to make calls only to other substations within the same group. Upon hearing dial tone, he dials the digit 3 of the call number and causes the selector switch 509, FIG. 5, to step to the third level at which the right normal post springs 512 operate make contacts 513. Closure of contacts 513 completes a path by which unique time division signals transmitted over a single conductor `cause electromechanical relays to operate and thereby control the operation of other circuits. The present signaling application follows the general principles typically shown -in FIG. 12 in that contacts 513 correspond to the send contacts and relay 519 corresponds to the receive relay. Furthermore time division signals are transmitted over the EC conductor path between the time ground outputs TG2 and the time battery outputs TB2. For the call being described, the path is traced from the group lidentification circ-uit 610 signaling terminal TG2 through the diode 6D4, make Ycontacts 5 of relay 6CU2, the back contacts 5 of relay 6CU3, back contacts 2 of relay 60N, back contacts 1 of relay 6LB, over the EC2 conductor extending by way of the trunk circuit 200, FIG. 3, to the corresponding EC2 conductor in the `transfer finder 491, FIG. 4, through contacts of relay 402 to conductor EC extending to the transfer relay group 493, further through back contacts 6 of relay 4S, back contacts 2 of relay 40N, extending over the EC conductor into the transfer selector circuit 509 and other equipment, not shown, to the upper winding of relay 519, and make contacts 513 of the right normal post springs to the signaling terminal output TBZ assigned to customer group number 1. Since no change has occurred in the relay 6CU2 in t-he identication circuit associated with the trunk circuit, it is obvious that identification in telligence has been transmitted to the selector switch for verifying that the present call is being directed to the same PABX customer group 1. Had the selector switch been dialed to other levels, either one or both of the normal post springs 511, 512 could have been operated but relay 519 would not operate because of -a'mismatch between the output terminals of the signaling arrangement. In the case of the call being described, relay 519 operates and extends the calling substation loop to an idle switchthrough type connector 515, FIG. 5. Upon switch-through of the transfer selector 509, ground is returned over the ON lead to relay 40N of the'transfer relay group, FIG. 4, Relay 40N operates and closes a time division signaling 12 path traced from terminal TG5 through the diode 4D7 and make contacts 2 of relay 40N, back contacts 6 of relay 4S, over the EC conductor of the transfer finder switch 491 to the EC2 conductor traced through FIG. 3 to the group identification circuit 610, FIG. 6, back contacts 1 of relay 6LB, back contacts 2 of relay 60N,

through the lower winding of relay 60N and the diode 6D1 to the TBS terminal. Relay 60N operates to the first of its two positions closes its X contacts, thereby closing make contacts 1 to the ONL conductor extending back through the tr-unk circuit to the ground which caused relay ZTR to fully operate through its lower winding.V

Fully operated relay 60N closes its make contacts 2 and completes .a pa-th between the EC2 conductor and the EC3 conductor of the identification circuit and also disconnects the path over which the signaling relay 519 in the transfer selector switch 509 was operated. It is understood that relay 519 remains operated over a locally completed circuit path, not shown. The connector switch l515 is ready to receive the last two impulsing digits 23 of the abbreviated call number 1323. In a known manner the connector rings the called second substration and ringing is cut off when the substation party answers the call. Having answered the call, reverse battery is returned through the selector 'banks and wipers 510 and Vthe -L from the rst PABX substation to a battery feed relay 4A,

in the relay group 493 and second the winding in the path through the d-iode 4D1 provides a holding path for the PABX connector impulsing relay not shown. Operated relay 4TA closes its make contacts 9 to relay 4TS. Relay 4TS operates its make contact 1 and closes a holding ground over the C1 conductor to the transfer nder switch 491. Operated relay 4A closes its make contacts 1 to relay 4B. A path is now conditioned for the operation of relay 4S to the rst of its two step positions and thereby closes its X contacts. This path is traced from ground standing on the C conductor of the transfer relay group 493 through make contact 1 of relay 4B operated, make contact 4, relay 4TA, and through the resistor 4R1 and the lower Winding of 4S to battery. Relay 4S operated only to its X contacts is prepared for the closure of a path by which booster battery, for instance, at the relay group 493 can cause the connector switch 515 to -be made switch-through. Relay 4B operated closes its make contacts. `6 `and 7 in preparation for the opera-tion of relay 4BS, which operate from make contact 5. Make contact 5 4also provides a holding ground to the H conductor eX- tending to the trunk circuit 200 since back contact 3 of relay 4BS now being operated will open the path to relay 4CC which had previously been `described as operated. At this time relay 4A of FIG. 4 is a battery feed to the calling PABX substation, the D relay, not shown, in the connector switch 51S provides battery feed to the called substation; and voice transmission between these substations is by way of capacitors 4C1 and 4C2 of the transfer rel-ay group, of FIG. 4. This is a local conversation path between the PABX substations, as it will be recalled X contacts and 40N; FIGURE 6, relays 60N, and

13 Limited conference call between first and second PABX substations and the originator of the did call The first-called substation engaged in the above-mentioned consultation call, may call in the outside party for a 3 way limited conference. He controls the setting up of a conference connection by merely dialing the digit 1, thereby causing relay 4A of the relay group 493 to fall back momentarily and close a path from ground through back con-tact 2, make contact 4 of relay 4BS, make contact 5 of relay 4TA and through the winding of 4CF to battery. Relay 4CF operates. Operated relay 4CF closes its make contacts 4 for a path from signaling output TG3 through diode 4D4, over the EC conductor switched-through to the ECZ conductor FIG. 3, which is closed by the group identification circuit 610 of FIG. 6 to the EC3 conductor extending back through the trunk circuit 20|) back contacts 11 of relay ZAT, FIG. 2, the lower winding of relay SCM, FIG. 3 and the diode 3D2 to terminal TB3. Relay 3CM operates to the first of its two positions and thereby closes its X contacts. This closes a path from ground through make contacts 9 of relay 2TR, make contacts X6 of relay 3CM, and the upper winding thereof to battery. Relay 3CM is thereby fully operated, closing its make contacts 4 and 5 for completing the transmission path to the trunk circuit 200 incoming conductors and the outside party who originated the direct inward dial call.

The second-called substation takes over the direct inward dialed call The first-called PABX substation, having set up the consultation and conference connections as described above, may relinquish control over the direct inward dialed call by merely replacing his handset. This causes battery feed relay 4A of the transfer relay group 493 to restore. Relay 4A restored opens the path to relay 4B, causing relay 4B also to restore. Relay 4B restored closes a path for forwarding a switch-through signal, booster battery for example, from the relay group to the switchthrough connector 515, FIG. 5. This path is from booster battery through make contacts 3 of relay 4TA, make contacts 1 of relay 4S, back contacts 2 of the same relay, back contacts 1 of relay 4B to the C conductor extending through the transfer-selector switch 509 yto the relays, not shown, in the switch-through connector 515. This booster battery signal causes the connector to switch the second-called PABX substation loop of the connector through the transfer-selector switch to the -L and -l-L conductors lof the transfer relay group, FIG. 4. Relay 4TA responds to this dry loop condition and restores its contacts. Relay 4S, previously operated to its X contacts, fully operates from the ground on the C lead of the relay group 493, over a path through X3 contacts of relay 4S, back contacts 4 of restored relay 4TA, and the lower winding of relay 4S to battery. Make contacts 5 of relay 4S provide a holding ground for the transferfinder switch 491. Release of relay 4B removes holding ground from the H and C conductor path extending to relays 2TR, FIG. 2, and relay 60N, FIG. 6. These relays' restore. Relay 2TR restored closes its back contacts 1 and 2 and sets up a closed path to relay 2E in the trunk circuit 200 from the loop path of the second-called substation taking over this call. Relay 2E reoperated, thereby causing relay 3B to operate as has been previously described, is now thefbattery feed and impulsing relay for the second-called PABX substation who may set up further transfer calls. Operated relay 3B closes make contacts 6 in a path traced from terminal TGS, the ECZ conductor connected with the EC conductor of the transfer relay group, FIG. 4, through make contacts 6 of relay 4S, make contacts 4 of relay 40N and through the lower winding of relay 4LH to terminal TBS. Relay 4LH remains operated through this winding after relay 4TS -has opened the path through the upper winding. Operated relay 4LH closes its make contacts 1 in a ground holding path for relay 4S and the C conductor extending to relays, not shown, in the connector switch. From what has been said above, it is lobvious that.fthe following relays will now restore: Relays SCM, SSW, 3T, 3P, 4B, and 4TS; also relay 60N of the group identification circuit, FIG. 6. The incoming fifth selector switch 500 and the switch-through connector 515 in FIG. 5, used in establishing the original inward dialed call, are no longer in use and will release.

As a reference, REFERENCE D, for the following call about to be described, the relays in operated condition are listed as following: FIG. 2, relay ZRV, ZRVS, 2A, 2E, ZAT to its X contacts; FIG. 3, relay 3B; FIG. 4, relay 4S fully operated, 4LH, and relay 40N; FIG. 6.. relay 6CU2.

The second PABX substation establishes connections tn a third substation In the course of the conversation with the outside party, the second PABX substation may call another substation referred to as the third substation, for consultation purposes.

This section of the specification points to the use of two transfer link switch trains, each including a transfer relay group 493, for establishing this consultation call. The first of the two trains is presently engaged in the call as just described, and the second transfer link 100 will be seized when the second PABX substation dials the prefix digit 1. In. this case the abbreviated call number is the prefix digit "1 plus a local directory number 324 of the party desired.

`On the basis of the above reference D listing the relays operated at this time, it is obvious that the dialing of the prefix l causes relay 2E of trunk circuit 200 to restore and later to reoperate. Relay 2E restored closes a path for the operation of the slow releasing relay 2C. This path is traced from ground at make contacts 2 of relay 2A, back contacts 1 of relay 2E, back contacts 9 of relay 3K, make contacts 5 of relay 3B, FIG. 3, and through the upper winding of relay TC to battery. With relay 2E reoperated and relay 2C not yet released a path is closed whereby relay 2TR operates to its X contacts. This path is from ground through make contacts 2 of relay 2A, make contacts 1 of relay 2E, back contacts 6 and 11 of relay 3K, make contacts 4 Iof relay 2C, over the TRD lead of FIG. 3 extending to a similar lead in FIG. 6 where the'path passes through back contacts 7 of relay 6CU3 to lead TRG, FIG. 3, through the back contacts 6 of relay 3P, and the upper winding of relay 2TR to battery. It is obvious that relay 2C operated has opened the transmission path to the outside party at its back contacts 1 and 2 and thereby prevented the prefix digit impulses from affecting the incoming connections to the trunk circuit 200. Before relay 2C restores its contacts, and particularly contact number 3, to set a marking on the vertical bank, not shown, of the transfer finder switch 491, FIG. 4, a guarding path is provided for holding relay 2TR operated to its X contacts. This path extends from ground at make contacts 2 of relay 2A through make contactsy of relay 3B to the left through back contacts 4 of relay ZAT, back contacts 3 of relay 2TR and the X4 contacts of the same relay to make contacts 3 of relay 3B, and the upper winding of relay 3P in series with the upper winding of relay 2TR to battery. This series holding path through relay 3P causes relay 3P to operate. Operated relay 3P makes a contact transfer at digit 0, also operates relay 3P and through its make contacts 6 completes a path for the operation of relay 2AT in the trunk circuit 200.

lrelay 4CC; and FIG. 6, relay 6CU2.

Restored relay 2C sets up a marking circuit to the transfer finder vertical bank, FIG. 4, and to the allotter circuit 492 shown on the drawing below the transfer finder 491. This marking path is from ground at make contacts 2 of relay 2A through make contacts 2 of relay 3B, back contacts 4 of relay ZAT, relay 2TR back contacts 3 and 4, through back contacts 3 of relay 2C to the SLM marking conductor extending to the vertical bank and allotter of FIG. 4. Described briefly, the allotter causes an idle transfer finder switch to search for the vertical bank marking and also the inward dialing trunk circuit 200 engaged in making this consultation call by way of the first mentioned transfer link 100 switch train Y which had established the connection to the second PABX substation. It isV recalled that the first transfer-finder switch 491 was held operated from the ground through make contacts of relay 4S over the C1 conductor extending back by way of make contacts 403 of the finder switch and the winding of relay 402 to battery. This holding arrangement Iof the first transfer-finder switch permits its banks and wipers to be used in setting up the consultation call -being described. After finding this trunk circuit, relay 402 of the second transfer-finder switch 491 operate `and switches through the transmission impulsing and control conductors from the first transfer-finder Ibank multiples 404 of FIG. 4 extending through the second transfer relay group 493 to the second transfer-selector switch 569yof FIG. 5 and its impulsing relay not shown. In response to seizure of the impulsing relay, the second selector switch returns a guarding ground on the C lead, FIG. 4, to the second relay group, thereby causing relay 4CC to operate over an obvious path. Relay 4CC operated closes its make contacts 1 for holding the second finder switch relay 402 operated. As in the case of the first described consultation call, relay 4CC operated closes its make contacts 3 and causes relay ZTR of the trunk circuit 200 to become operated through its X contact '7. Itis obvious that the operating ground path causing the operation of relay ZTR can also be traced back through the group identification circuit 610 of FIG. 6 and specifically the ONL conductor. Fully operated relay 2TR disconnects relay 2E from the previously described path extending back to the second'PABX substation loop, disconnects the marking ground to the transferfinder switch banks, 404, and also provides a holding Y V ground for relay'SP of the trunk circuit.

The relays in an operated condition are summarized, REFERENCE E, as follows: FIG. 2, relays 2RV, ZRVS, 2A, 2AT to its X contacts, and 2TR fully operated; FIG. 3, relay 3P; FIG. 4, first transfer :relay group, relays 4S, 4LH, and 40N; FIG. 4, second transfer relay group, Y At this point in the establishment of the consultation call, the second transferselector switch 509 returns dial tone and is ready to accept digital impulses of the called number, that is 324. The group identification and time division signaling techniques now effected as the second PABX substation controls the establishment of the consultation call connections, by wayY of the first and second switch trains, to the third substation of that PABX group 1 is generally the same as described above. This refers to the portion of the operational description following the REFERENCE B listing of operated relays in the section relating to the establishment of the rst consultation call connections from the first PABX substation to the second substation.

Upon establishment of the present consultation call, the relays in operated condition and listed, as REFER- ENCE F, according to figures of the drawings are as follows: FIG. 2, relay 2RV, 2RVS, 2A, ZAT to its XV contacts, and ZTR; FIG. 3, relay 3P; FIG. 4 first transfer relay group, relays 4S, 4LH, and 40N; FIG. 4 second transfer relay group, relays 4BS, 4A, 4TA, 4B, 4TS, 4S, to its X contacts, and 40N; FIG. 6, relays 60N, and 6CU2.

The third-called substation disconnects from the consultation call The third-called substation party, engaged in consultation with the second substation party, may restore his handset and thereby release -the connector'by which he was called. Connector switch 515, FIG. 5, restored removes the battery feed by which the relay 4TA of the second transfer relay group 493 had remained operated. The second transfer-selector switch and relays are now restored and ground is removed from the ON conductor back to the relay group, thereby causing relay 40N to also restore. From what has been said above in regard to the holding paths for the transfer relay group and the transfer finder switch, it is obvious that relays 4BS, 4A, 4B, 4TS, and 4S, of the second transfer relay group now restore, as well as the second transfer switch, FIG. 4. Relay 4B restored opens its make contacts 5 and removes the holding ground which permits relay 2TR of theV trunk circuit 200, FIG. 2, and relay 60N of the identification circuit 610, of FIG. 6 -to restore. Relay 2TR restored reconnects relay 2E to the path previously traced to the second PABX substation loop. Relay 3B, FIG. 3, of the trunk circuit reoperates, but relay V3P restores.

At this time the relays in an operated condition are summarized, REFERENCE G, as follows: FIG.V2, relays 2RV, ZRVS, 2A, 2E, ZAT to its X contacts; FIG. 3,V

relay 3B; FIG. 4, first transfer relay group, relays 4S, 4LH, and 40N; FIG. 6, relay `6CU2.

.flntornanz'cally forwarding a directory listed number call t0 a PABX attendant This portion of the specification relates to call forwarding arrangements, in the Vswitching system serving groups of private .branch exchanges, wherein the inward dialed connection set up to a PABX group directory number, and terminating in a listed number trunk 516 rather than to individual substation numbers of the PABX, auto-V n two digits 22, the connec-tor switch 515, FIG. 5, established connections with the called PABX substation, while in the present case the last digits 25 will cause the connection to terminate on the listed number trunk circuit 516, FIG. 5. Upon seizure of the listed number trunk reverse battery is returned to the calling connection for a period of time less thanfor the inward dialed call to the substation. In view of the above, is evident that the inward dialing and PABX group marking techniquesV already described in detail in connection with the establish-ment of a call to a substation also applies to the listed number call. The present description will therefore diverge from the point at which relay 2SP operated, transfers etc. A recapitulation of the relays in an operated condition is as follows: FIG. 2, relay 2SP; FIG. 3, relay 3CM; FIG. 6, relay 6CU2.

Relay 2SP operated, transfers, at its make contacts 1 and 2, the incoming conductors from a physical connection through the incoming selector 500 4to a connector switch 515 to the upper and lower winding of relay 2A of the trunk circuit 200. Relay 2A operates. Itis obvious that relay 2A provides normal polarity battery feed through its windings to the incoming connection conductors until relay ZRV operates to reverse the polarity and provide answer supervision to the incoming connection. As in the case of the previous description of an inward dialed call, relay 2A operated closes its make contacts 2 and opens S break COIl'ET-S 3 and sets up a divided circuit condition whereby the charging current of capacitor 2CA5 through the lower winding of relay ZRV opposes the current through its -upper winding. A path is also closed so that relay ZAT operates to its X contacts as previously traced. In the case of the present listed number call being described listed number trunk circuit 516 of FIG. now removes reverse battery from the path traced .back to the relay 2SP of the trunk circuit 200, FIG. 2, thereby allowing relay 2SP to restore. Restored relay 2SP opens its make contacts 5 and thereby prevents relay ZRV from operating through its upper winding. Relay 2SP restored opens its make -contacts 1 and Z and thereby permits relay ZA to also restore. Restored relay ZA closes a path by which relay ZAT becomes fully operated to its second position. This path is traced from the ground back from the incoming switch train over the CSW conduc-tor of FIG. 3 extending back to FIG. 2 through back contact 6 of relay ZRVS, back contact 1 of relay 2A, back contact of relay 3K, extending to back contact Z of relay ZRV, back contact 5 of relay ZA, through the thermal element device ZTRl, diode ZDS make X contacts 10 of relay ZAT and through the lower winding of that relay to battery. After a period of time characteristic of the thermal element ZTR1, relay ZAT becomes fully operated. Operated relay ZAT closes its make contacts 1, 2 and 3, and thereby closing the paths over which relay 2A is again operated, and another path by which ringback tone is returned to the party m-aking this listed number call. The trunk circuit 200 is now conditioned for forwarding the inward dialed call to the PABX attendant of customer group number 1 only as the group identication relay A6CU2, identifying that group, remains operated over the LK conductor extending to the trunk circuit.

As an interjection at this point in the description of the listed number call, all attendants of customer group number 1 may be engaged in other calls and therefore unable to accept the call being made. In other words the gating circuit arrangement with the customer start circuit would be closed, that is relay 6GC of the customer start circuit 620, FIG. 6, would remain operated and hold its contacts 2 yand S open. When an attendants position 900(1) of this group became idle and could accept this call relay 6GC would restore and close a path by which relay 3GT, FIG. 3 of the trunk circuit 200 operated. This closed path is traced from ground through back contacts 2 of relay 6GC over the GT conductor through make contacts 4 of relay 6CU2 of the identication circuit 610, back contact 4 of relay 6CU3, the GT conductor extending to the trunk circuit, FIG. 3 where it continues through back contacts 4 of relay SGT, back contact 5 of relay SCO, make contact 5 of relay ZAT, and through the winding of relay 3GT to battery. Relay SGT operates, and remains temporarily held through its make contacts 4. Its make contact 3 closes an operating path over the GP conductor extending through the group identification circuit, FIG. 6, to the customer start circuit relay 6ST. Operated relay 6ST closes its make contacts 1 for a path over the M conductor to the link allotter 720, contacts 704 through other equipment, not shown, back contact 711 of relay 710 and through the rotary magnet winding 7MM, FIG. 7 to battery. Interrupter contact 709 of the rotary switch cooperate with the relay 710 and its back contacts 711 in such a way that the rotary switch steps until it nds the position circuit 900(1) of the attendant 1000(1) free to accept this call. This position circuit is found at the point on rotary switch level A at which a path is closed from the ground at make contact 3 of relay 6ST, FIG. 6 over the N conductor extending through the winding of relay 70S of the link allotter 720, FIG. 7, other equipment not shown, through the A level bank and wiper set to the H conductor extending to the position circuit of FIG. 9, where the path continues through break contacts 1 of relay 9PBA, over the PB conductor back to the customer start circuit, FIG. 6, through make contacts 4 of relay 6ST and resistor 601 to battery. Relay 703 of the link allotter, FIG. 7 operates and opens its contacts 704 to open the self interrupting circuit to the rotary switch 7MM. Having found the idle attendants position circuit, one of the four assigned to customer group number 1 in this case, the rotary switch wipers come to rest in the position shown in the link allotter, FIG. 7. Ground potential through other equipment, not shown, extends through level C wiper and bank contacts to the PM conductor extending to the position finder 750 through the lower winding of relay 713 to the vertical bank contact 702 thereby providing a marking at the bank levels at which the position nder switch can establish connections from the trunk circuit 200 to the position circuit 900(1). Rotary switch level D extends a ground through other equipment not shown over the TM conductor extending back through the group identification circuit 610, FIG. 6 to the similar conductor in the trunk circuit, FIG. 3 where it passes through make contacts Z of relay SGT, now operated, to the vertical bank conductor VB, and to the trunk finder 730 vertical bank contact 714. Having marked the vertical bank level multiples of the position finder and trunk nder equipment, as described above, the link allotter 720 allots an idle position iinder 750 and an idle trunk inder 730 to iind these markings respectively. The link allotter controls the operation of the rotary switch 7MM1 so that its levels A1 and B1 become associated with the allotted trunk finder and position finder switches, respectively. The position nder switch steps vertically under the control of the position iinder control circuit 715 until the vertical bank wiper associated with conductor 700 engages contact 702. At this time a circuit for relay 713 of the position iinder switch is completed over a path traced from the marking ground on vertical bank contact 702 over the conductor 700 extending through the position finder control relays and the B1 level of the rotary switch 7MM1 to other equipment not shown to battery. Relay 713 operated controls a rotary hunting operation of the position finder until an idle terminating circuit 800(1) of FIG. 8 is found on that level. As mentioned above, the link allotter controls the operation of the trunk nder causing it to step vertically until its vertical bank wiper cornes to rest on the marking contact 714 and closes a path further traced through the conductor 706, the lower winding of relay 712 to the link allotter rotary level A1, other equipment, not shown, to battery. Relay 712 controls a rotary hunting operation of the trunk finder 730 until a switch be comes associated through its bank contacts and wipers 707 with the trunk circuit 200, forwarding this listed number call to the attendant 1000(1). With their finding operations completed the position-tinder and the trunk-finder form a call-forwarding concentrator link directly connecting the terminating circuit 800, FIG. 8 to the inward dialing trunk circuit 200, FIG. 3, by way of conductors -F, -l-F, CF and EC. The switch through link completes a path over which relay SLB of the terminating circuit now operates. This path is traced from the link allotter 720, FIG. 7, by Way of rotary bank F traced from ground through other equipment, not shown, extending over the RM conductor through the group identification circuit 610, FIG. 6 to the trunk circuit 200 with the same conductor extending through make contact 1 of relay SGT operated, the upper winding of relay SCO, make Contact 4 of relay 2A operated, to the CF conductor extending through the link path to the terminating circuit 800(1) in FIG. 8 by way of back contacts 2 of relay SAN and the lower winding of relay SLB to battery. Operated relay SLB closes its make contacts 4 Ifor a path traced from battery through other equipment, not shown, back contact 7 of relay SAN, make contact 4 of lrelay SLB, back contact 2 of relay CS1, back contact 3 of relay CS2, to the CS1 conductor extending to the turret equipment 1000( 1), specically lamp 1070 to ground which indicates that this is a listed number call being forwarded to the attendant. Operated relay SLB also closes a path for a time division signal which indicates to the attendant the 

1. IN A TELEPHONE SYSTEM, A CENTRAL EXCHANGE HAVING: IN COMING CONNECTIONS TERMINATED BY A TRUNK CIRCUIT; AUTOMATIC SWITCHING EQUIPMENT INCLUDING CONNECTOR SWITCHES: AN ATTENDANT''S POSITION; GROUPS OF SUBSTATIONS CONNECTED TO SAID CONNECTOR SWITCHES AND ARRANGED TO RECEIVE CALLS INCOMING TO SAID EXCHANGE OVER SAID CONNECTIONS AND SAID TRUNK CIRCUIT BY WAY OF SAID SWITCHING EQUIPMENT BOTH UNDER THE LISTED NUMBER OF SAID GROUP AND UNDER THE NUMBER INDIVIDUAL TO THE SUBSTANTION; AND A LISTED NUMBER CIRCUIT ALSO CONNECTED TO SAID CONNECTOR SWITCHES; SAID LISTED NUMBER CIRCUIT INCLUDING: MEANS OPERATED UPON SEIZURE OF SAID LAST-MENTIONED CIRCUIT BY ONE OF SAID CONNECTOR SWITCHES FOR AUTOMATICALLY RETURNING TO SAID TRUNK CIRCUIT AN ANSWERING SIGNAL OF LESS THAN A PREDETERMINED DURATION; AND SAID TRUNK CIRCUIT INCLUDING: ANSWERING SIGNAL RESPONSIVE APPARATUS; TIMING MEANS; AND MEANS JOINTLY CONTROLLED BY SAID APPARATUS AND SAID TIMING MEANS FOR RETURNING A SUPERVISORY SIGNAL TO SAID INCOMING CONNECTIONS IF SAID ANSWERING SIGNAL IS OF MORE THAN SAID PREDETERMINED 